Slovenian soldiers build a razor wire fence on the Slovenian-Croatian border in Gibina, northeastern Slovenia, on November 11, 2015. Slovenia's army began on November 11 rolling out razor wire along the border with Croatia, in a move billed by the government as designed to better manage the influx of migrants. Photo: AFP/Jure Makovec

Sweden and Slovenia on Wednesday became the latest European nations to tighten borders to ease an unprecedented migration crisis as African leaders warned their EU counterparts against building a "fortress" Europe.

The moves came as more than 50 European and African leaders met at a summit in Malta aimed at agreeing a joint strategy to deal with the biggest flow of refugees and migrants since World War II.

Sweden's decision to reinstate border controls for 10 days from Thursday and Slovenia's move to roll out razor wire on its border with Croatia underlined again the divisions within the 28-nation bloc over how to respond to the crisis.

After Hungary sealed its borders last month, Slovenia found itself on the main Balkans route for the thousands of migrants who are landing in Greece every day after braving the short but dangerous sea crossing from Turkey.

EU leaders and officials have expressed fears that unilateral decisions to stop migrants will lead to the collapse of the borderless Schengen zone, a pillar of a united Europe.

Austria and Germany, both Schengen countries, have in the past months reinstated border controls as they struggle to cope with the flow of asylum seekers from the Middle East, Africa and South Asia.

African Union chief Nkosazana Dlamini-Zuma told the leaders gathered in a 16th century stone building in Valletta that it was in Africa's interest to prevent its youth from leaving.

"We have to industrialize and modernize our continent otherwise young people will continue to go elsewhere," Dlamini-Zuma said.

But she added: "The problem we are facing today is in part because some countries in Europe have taken a fortress approach."

European nations aim to boost cooperation with African countries to protect legitimate refugees, send home irregular migrants and stop those who smuggle them, while offering Africans expanded legal channels of migration.

As a carrot, the European Commission, the bloc's executive arm, is setting up a 1.8-billion-euro "trust fund" for Africa and has urged member states to match that sum -- although European sources said it was not sure that they would.

The money would go towards tackling the root causes of migration, such as poverty and conflict.

European Commission President Jean-Claude Juncker renewed appeals for member states to match the funds while Senegal's President Macky Sall, who chairs the Economic Community of West African States, issued a similar call.

"We would like the amount available to be increased," Sall said.

Progress on the main thrust of the EU's current migrant strategy -- fostering cooperation with Turkey -- will be discussed when EU leaders meet without the Africans on Thursday in Valletta.

Turkey has overtaken North Africa as the main launching point for migrants coming to Europe, and currently hosts two million Syrian refugees.

But tensions with Ankara have undermined efforts to get it to crack down on the huge numbers leaving its shores, many of whom have drowned.

At least 18 people, including seven children, perished in sinkings off Turkey on Wednesday, adding to a total of more than 3,400 people who have died trying to reach Europe by sea this year.

'Outsourcing of rights abuses'

The EU-Africa summit was first called months ago when the Mediterranean route from a lawless Libya was still the main springboard for migrants travelling to the EU in battered fishing boats and flimsy dinghies.

Nearly 800 migrants died in a single shipwreck off the Libyan coast in April.

Amnesty International's Iverna McGowan raised fears that the summit would reinforce the idea of a "Fortress Europe," telling AFP the approach "can lead to an outsourcing of human rights abuses and is quite worrying."

Rights groups have also voiced concerns about what they see as a trend towards EU governments making aid contingent on African countries agreeing to take back economic migrants.

Africa accounts for about 140,000 of the nearly 800,000 migrants who have reached Europe by sea this year, according to figures from the International Organization of Migration.